Fungi in Mulch

by Karen Weiland, Advanced Master Gardener

Mulching2Each spring I like to spread a fresh layer of mulch on my flower beds.  As you may know mulch helps conserve moisture in the soil, keeps weeds at bay, keeps the soil temperatures lower in the summer and insulates the plant roots in the winter. Usually, organic materials (mulch) inhibit unwanted microorganisms like soil borne pathogens that cause diseases in plants and organic materials can stimulate the activity of many types of beneficial microorganisms such as mycorrhizal fungi. I have found that sometimes mulches can contain microorganisms (primarily fungi) that can become a nuisance and cause certain diseases of some plants. The type of organic matter from which it was produced and the degree of decomposition and treatment before it was used in the landscape largely determines whether a mulch will have beneficial or detrimental effects.

Stinkhorn Fungus

Examples of nuisance fungi would be shotgun or artillery fungus, slime molds and dusty mass fungi.  Shotgun fungus shoots its spore masses high into the air.  These spore masses will stick to about any surface and looks like tiny tar spots on plant leaves or

Artillery Fungus on Siding
Artillery Fungus on Siding

house siding.  They are very difficult to remove, if you can at all.  They may leave a permanent stain.  Slime molds first appear as bright orange or yellow masses that vary from several inches to a foot in width.  They produce tiny spores that with time will dry out and blow away.  I have had a fungus called stinkhorn in my mulch.  It has a long, orange, upright cylindrical body with a brown cap which can give off a nasty odor.  When mulches are applied too deep (4-6 inches), especially if they are prepared from fresh woody materials, a substance can occur that is a water-repelling moldy chunk of material.  I also have some of this on my mulch.  Water is easily repelled and a dusty product is released.  The ideal depth of mulch is 1 ½ to 2 inches.

Mycorrhizae, a beneficial fungi, is stimulated by the slow release of organic sources of nitrogen and carbon in organic matter when applied in a shallow layer.  However, a deep layer (4 to 6 inches) of organic matter can inhibit the development of mycorrhizae.  Mycorrhizae are very important in the maintenance of healthy plants.

Verticillium dahlia
Verticillium Dahlia

Fresh mulches prepared from trees killed by plant diseases can harbor plant pathogens.  Verticillium dahlia, which is a fungus that causes wilts and death in trees and shrubs, can be harbored in infested mulch and then kill plants in the landscape.   Short-term (six weeks)  composting under high temperature conditions (130 to 160 degrees) is sufficient to kill most plant pathogens and thus avoid problems.

The pH or acidity of mulch is another important factor.  Sour, smelly mulches may have a pH range of 2.5 to 4.8.  Bacteria that inhibit fungal growth cannot materialize when the pH is less than 5.2.  Highly acidic materials are toxic to most plants and will promote the growth of the dreaded detrimental fungi.

Sometimes not much can be done to avoid nuisance fungi.  I just chop mine up with a shovel or hoe and give it a good soaking of water.  You can kill nuisance fungi by removing the mulch from the landscape, place it in a heap after wetting it down thoroughly and then wait for the self-heating to occur.  Fresh, finely ground woody material should be avoided unless it has been composted before using.  Fresh, course woody material is less likely to cause problems unless it is applied too deep.

As always, Happy Gardening!

More information about gardening and related subjects is available online at  The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange Co.,  636-2111 in Noble Co., 925-2562 in DeKalb Co. and 668-1000 in Steuben Co.